1. Field of the Invention
The present invention relates to a wide-range fluid flow measuring device and, more particularly to an adjustable laminar flowmeter which maintains low Reynolds numbers for a wide range of flow.
2. Description of the Prior Art
In general, laminar flowmeters which use the linear relationship between the flow rate and some characteristics of the fluid, such as the change in static pressure or rate of heat transfer, are known. The details of their operation are well known to those of ordinary skill in the art. As is also known, to produce and maintain laminar flow through such a measuring device certain criteria have to be met. One of the most important criteria is the relationship between mean fluid velocity and certain dimensions of the fluid passage. Extensive studies have been conducted to characterize these properties and the most widely used characterization is referred to as the Reynolds number. The Reynolds number is a dimensionless parameter and is based on the results of an investigation of fluid flow in pipes in 1883 by Osborne Reynolds (and subsequent experimentation). Based on this work, it has been determined that for smooth pipes the transition from a laminar to turbulent boundary layer occurs when the ratio of ud.rho./.eta.a becomes larger than approximately 2000, where u is the mean velocity of the fluid, d is the characteristic linear dimension of the pipe, .rho. is the density of the fluid, and .eta.a is the absolute viscosity of the fluid. If the above condition can be maintained for the flow range of interest through the measuring device, the advantages of the linear relationships discussed above can be realized. For example, for laminar flow, the radio between the static pressure drop and the fluid flow rate is linear. The ratio between heat transfer and the fluid flow rate is also linear. A more detailed discussion of the characteristics and requirements of laminar flow will not be provided here since such information is well known in the art.
In general, a laminar flowmeter may comprise a first flow path (or paths) for a main body of flow and a second flow path (or paths) which may be used for purposes of measuring one or more flow characteristics. The first flow path may correspond to a main (undiverted) flow and the second flow path may correspond to a secondary (diverted) flow path. Preferably, there is a selectable predetermined ratio between the volume of diverted to undiverted flow. An advantage to be obtained by maintaining laminar flow is that the flow rate can be calculated based on only a small portion of the fluid by diverting that portion through the measurement passage (e.g., the second flow path) and applying a constant factor corresponding to the ratio of diverted to undiverted flow. This eliminates the need for look-up tables to provide the actual flow rate versus an indicated output. However, it is also desirable to be able to control the ratio of flow in the main and secondary flow paths. Attempts have been made to enable control over this ratio of flow, but they have not been completely satisfactory.
In high-volume manufacturing, where an objective is to mass produce parts with consistent dimensions, the cost vs. accuracy ratio becomes significant, especially when the tolerances are tight as in the case of the fourth power dependency of pressure drop to a pipe radius. Therefore, it is desirable to be able to mass produce the basic flow element by molding, for instance, and make provisions for adjustment at a later stage.
Many flow measuring devices have attempted to maintain low Reynolds numbers to take advantage of the linear relations discussed above to enable a simple straight-line approximation to calculate the flow rate. One example of such a device is disclosed in U.S. Pat. No. 3,838,598 issued to Tompkins. Tompkins teaches using a plurality of capillaries to create laminar flow through the measuring device and using differential static pressure information to calculate the flow rate. However, capillary flow pipes are difficult to efficiently mass produce and once fabricated, they do not easily lend themselves to adjustment in the effective diameter. Another approach is found in U.S. Pat. No. 4,118,973 issued to Tucker et al. In Tucker, grooves of rectangular cross section are machined into plates and by varying the width of the channels and/or by stacking a number of plates, the effective diameter of the element can be varied while low Reynolds numbers are maintained. Alternative designs are proposed by Jouwama in U.S. Pat. No. 4,427,030 and by Mermelstein in U.S. Pat. No. 4,497,202, where the laminar flow element is placed in the bypass and only a small portion of the flow is diverted through the primary passage for measurements. Adjustment of the diversion ratio in flowmeters of this type is achieved by changing the plates that make up the bypass. This is also inefficient. Another design for an adjustable laminar flow element is disclosed by Korpi in U.S. Pat. No. 4,800,754. In Korpi, one cylindrical piece with multiple longitudinal grooves replaces the stacked plates used by Jouwama and Mermelstein. These grooves are blocked off by the webbing on the input side and coarse adjustment in the diversion rate is achieved by the removal of the webbing. A mechanism for fine adjustment in the diversion ratio is also provided. U.S. Pat. No. 4,461,173 issued to Olin discloses a multirange flowmeter in which a multi-position valve is located in the primary passage to divert all or a portion of the flow to the secondary passage.
An adjustable laminar flow bypass which lends itself to field adjustment is disclosed by Drexel et al. in U.S. Pat. No. 4,524,616. Drexel discloses a frusto-conical adjustable laminar flow bypass restrictor within a conically tapering bore to form a conduit which is allegedly capable of maintaining laminar flow and which is said to be adjustable to form annuli of varying thicknesses to allow proper sensor calibration over a wide range of flow rates. At Cols. 1-2 of Drexel, a summary of certain considerations relevant to adjustable, laminar flowmeters is provided.
However, certain drawbacks exist with respect to Drexel. For example, in Drexel, restrictor 72 is secured by a coiled spring member 110 which abuts shoulder 52. Procession and regression of the restrictor in the direction of flow causes a change in the thickness of annular gap 108. In this arrangement, the character of the flow is a function of the cube of the gap thickness. As stated in Drexel, this means that the perimeter surface of the flow restrictor and the surface of the frusto-conical bore should be as concentric as possible to ensure even laminar flow throughout the range of adjustment.
Several problems occur with this adjustment scheme. One is the inability to achieve satisfactory adjustment resolution for lower flow rates. In part this is due to the fact that the adjustment mechanism is not a linear function of the number of screw turns. Another is that turbulence may be created if certain precautions are not taken. Additionally, precise manufacturing tolerances must be set to ensure linearity over a wide range. Moreover, it appears that the adjustment screw is positioned such that an adjustment tool has to be inserted through either the inlet or outlet of the flowmeter. This means the flowmeter may need to be removed to readjust it. Various other drawbacks also exist.